The establishment of a subsea well, for example a petroleum well, is conditional on a wellhead, which is arranged on the seabed, being stabilized in such a way that strain, for example lateral forces, to which the wellhead Christmas tree is subjected because of currents in the surrounding water masses is transmitted to the seabed masses surrounding the top part of the wellbore. The stabilization is typically provided by a conductor casing, which defines the top part of the wellbore against the surrounding unconsolidated masses, being cemented against the unconsolidated masses after, for example, the conductor casing has been lowered into a drilled hole or been driven down into the unconsolidated masses by an impacting device. The conductor casing forms a low-pressure barrier in the well and is connected to a low-pressure housing in the wellhead. Through the conductor casing and further through an established borehole a well pipe that forms a high-pressure barrier in the well is extended.
An improved stabilization is achieved by the wellhead being provided with a well frame, which is supported on the seabed. The applicant's own suction foundation (Conductor Anchor Node=CAN) as described in NO 313340 B1 and the corresponding US2003029620 A1 provides a larger contact area between the upper part of the conductor casing and the surrounding seabed mass in order thereby to further increase the stability of the wellhead. Preferably, the conductor casing is extended through a supporting pipe, which is typically centered in the suction foundation and secured to the top cover of the suction foundation and to a lower portion of the skirt of the suction foundation.
The establishing of a wellhead is laborious and especially the installation of a conductor casing may result in unconsolidated masses around the conductor casing and wellhead foundation being washed out. The risk of such washouts occurring depends on several factors, among them the properties of the unconsolidated masses. Measures are therefore needed in order to reduce the risk of such washouts.
There is also a need to be able to establish several wellheads on the same foundation without this resulting in the foundation having to have a size, which makes the installation unduly demanding because of the dimension and weight.
WO2015054766 discloses the installation of conductor casings, that is to say a low-pressure barrier, in a seabed, with an assembly of one or more suction foundations with one or more integrated pipes extending in the full height of the suction foundation, inside or on the outside of the suction foundation(s) and projecting up above the top of the suction foundation(s). The suction foundation(s) is/are provided with top cover(s) closing the top(s) of the suction foundation(s).
WO2015118348 discloses an apparatus for installing a wellbore in a seabed, in which the upper end of the suction foundation is provided with a wellhead for engagement with a conductor casing, that is to say a low-pressure barrier, extending through the housing of the suction foundation.